Nitrox Only

[debajo agua]

Happy Thanksgiving,

Recently, I bought a couple of tanks that had stamped on the valve stem "Nitrox Only". Can anybody provide a good reason why I should only fill these tanks with Nitrox?

Debajo

 

[Jim Lapenta]

Nope, no good reason whatsoever. And BTW Air is 21% Nitrox. Some marketing gimmick I suspect to get more money for them when sold new.

 

[al248005]

You can use them for reg air fills but once you switch the tank to regular air you can't switch it back to nitrox unless its O2 cleaned. Both of my tanks had nitrox stickers on them when I bought them but I switched them over to regular air (just peeled the stickers off).

PS...The stickers are a PITA to get off if they are all scratched up.

 

[rongoodman]

You can use them for reg air fills but once you switch the tank to regular air you can't switch it back to nitrox unless its O2 cleaned. Both of my tanks had nitrox stickers on them when I bought them but I switched them over to regular air (just peeled the stickers off).

Not necessarily. They only have to be O2 cleaned if you LDS is using partial pressure blending, which involves added pure O2 to the tank. If he's using a stick or membrane blending system, or has banked nitrox, they can be used for nitrox as is. They would have to have the contents labeled, which might or might not involve one of those gaudy Nitrox bumper stickers, depending on the shop.

 

[debajo agua]

You can use them for reg air fills but once you switch the tank to regular air you can't switch it back to nitrox unless its O2 cleaned.

Any idea why that is? I can't imagine air contaminating a O2 cleaned (Simple Green cleaned) tank.

 

[boulderjohn]

This is an area of great confusion. I believe a lot of it has to do with old thinking that has become outmoded. Remember that there was a time when no recreational agency allowed the use of nitrox, and there was a time that no presentations on nitrox were allowed at the annual DEMA convention. It was considered too dangerous. Now even OW students can use nitrox on their 4th certification dive.

I just bought two new tanks, and I read the warnings that came with the valve carefully. I noticed the care with which they were worded. In their totality, it was clear that the valves came to me ready to be used for a pure oxygen fill, although it did not say so specifically. What it did say was that if you use the valve on a tank being used for that purpose, make sure the tank is properly cleaned.

So what about filling a nitrox tank with air? The fear is that if you do use it with air, the air will contain contaminants that could ignite in the presence of pure oxygen. Pure oxygen is introduced into a nitrox tank if it is filled via the partial pressure method. The fear is that if you just hook the tank up to the compressor and fill it with air, it will thus become contaminated and create a danger for the filler. Let's look at the steps of the partial pressure blending method for nitrox to see the problem with this line of thinking.

Step 1: Add oxygen to tank
Step 2: Hook the up to the compressor and fill it with air.

So how is it that hooking the tank up to the compressor to fill it with air introduces contaminants if that is all you are doing, but it doesn't introduce contaminants if oxygen was added first?

The answer is it doesn't. There are different qualities of air, with different requirements for filtering out contaminants. If the compressor you are using is filtered enough to create nitrox via partial pressure, then it is pure enough to give you an air fill without the need for oxygen cleaning afterwards. I have no qualms about putting air in my tanks.

---------- Post added November 28th, 2013 at 09:59 AM ----------

Any idea why that is? I can't imagine air contaminating a O2 cleaned (Simple Green cleaned) tank.

As mentioned above, it depends upon the level to which the air is filtered.

Imagine if you could put in the air typically found in Los Angeles without a good filter!

 

[Wookie]

There is air, and then there is air. I wouldn't mix grade e air with oxygen, but then, I don't have to. I verify my cylinders O2 clean or clean them every year. All 75 of them. Then I fill them with O2, or nitrox, or trimix, or oxygen compatible air without regard for what was in it the last time. If you keep all of your cylinders clean, and only fill them from known clean sources, the stay clean.

 

[DevonDiver]

Any idea why that is? I can't imagine air contaminating a O2 cleaned (Simple Green cleaned) tank.

As Wookie explained, the air used for blending nitrox is filtered to a higher standard (reduced contaminants to reduce explosion/fire hazard). Regular compressed air (not filtered to the standard for nitrox blending) is not deemed sufficiently clean. If used, there exists a risk that 'regular' compressed air could introduce contaminants to the cylinder that present a risk if pure oxygen is subsequently added to the tank (as is the case when Partial Pressure blending).

Were the tanks purchased second-hand? If so, it is possible that the previous owner added the marking to clearly identify that their tanks should not be filled from an unsuitable source (wishing them to remain O2 clean for nitrox use at any vendor).

 

[debajo agua]

If I'm correct, then the point is as follows: This is a warning left over from a by-gone era, when tanks could be filled with sub-standard air (without contaminate and oil filters) and consequently, there was an concern of the risk of fire or explosion... Or Today, this might be a issue in a country that has less-than-adequate regulation in relation to tank fills.

 

[Hatul]

Is the reason that oxygen is added first because O2 tanks are only filled to 2000 psi?